Crystal grinding and etching



Oct. 16, 1945. H. F. FRUTH CRYSTAL GRINDING AND ETCHING Filed June 13; 1945 2 Sheets-Sheet l 2N 60m wu INVENTOR HAL F. FRUTH f BY ATTORNEYQ Oct. 16, 1945.

- H. F. FRUTH I CRYSTAL GRINDING AND ETCHING Filed June 15, 1945 2 Sheets-Sheet 2 N. 0 h. 5658 m9:

GRINDING TIME INVENTOR HAL F FRUTH BY ATTORNEYS Patented Oct. 16, 1945' CRYSTAL GRINDING AND E'ICBING Hal F; Fruth, Chicago, Ill., assignor to Galvin Manufacturing Corp corporation of Illinois oration, Chicago, IlL, a

Application June 13, 1945, Serial No. 599,152

12 Claims.

The present invention relates to improved methods of producing small fragile crystalline objects or bodies, and more particularly to improved methods and apparatus for producing" piezoelectric crystals to impart predetermined resonant frequency and activity characteristics thereto and to provide an improved crystal structure having more stable operating characteristics. This application is a continuation-inpart of copending applications Serial No. 526,700, filed March 16,1944, Serial No. 479,928, filed March 20, 1943, and Serial No. 544,673, filed July 13, 1944.

In the manufacture of small crystalline arti-' cles or parts it is frequently desirable; if not essential. that certain face surfaces of ach article or 'part be ground to exact dime ions and exact contour, either for the purpose imparting desired operating characteristics to thepart or 'to satisfy the structural requirements of the particular device in which the part is to be used.

Thus in the manufacture of piezoelectric crystals, such for example, as quartz crystals, adapted for use in communication circuits and, more particularly, for use in crystal microphones, radio transmitting and receiving systems, and the like, the crystal blanks are first cut from the crystal,

stock and are then finished to the dimensions required to provide the desired frequency and activity characteristics. The usual crystal blank is cut in the form of a wafer-like rectangular piece having dimensions slightly larger than the desired dimensions. Each blank is thenreduced to the approximate rectangular dimensions desired by lap grinding the sides and'ends of the crystal on a suitable grinding wheel. a All finish grinding operations are conventionally performed by hand, both the edge and face .grinding operation, periodically tested to determine the frequency characteristics thereof.

The above-described methods now commonly employed to finish grind piezoelectric crystals are .tediously slow, require skilled labor and hence are costly from the labor standpoint, and require the use of exceedingly 'flne abrading materials, such, for example, as optical powders and diamond dust. Moreover, the crystals obtained by using these methods are not entirely satisfactory in operation, particularly when operating under widely varying temperature conditions. The operating diiliculties that have been experienced are largely attributable to the fact that the manual and semi-mechanical methods heretofore used are essentially high contact pressure grinding methods, with the result that discontinuities, in the form of scratches, hills and valleys, filled up pores and cavities, and the like, are produced in the crystal blank surfaces as the grinding "action proceeds. Also, in the finish grinding of crystals for use in ultra-high frequency circuits. the thickness of each crystal blank is limited to a value of approximately 7 mills or less. As a consequence, high contact pressure methods. of

ing operations being performed by bringing the desired surface or edge to bear against an abrasive surface and manually moving the bearing surface of the crystal across the abrading surface. For example, one very common practice is that of pressing the crystal face against the abrasive surface bylmeans of a finger held against the opposite face of'the crystal and moving the crystal bodilyo'ver the abrasive surface. in an orbit roughly approximating a figure 8.

An alternative method commonly in use, is that of supporting two or more crystals on a flexible supporting strip, such as rubber, cork or the like, and moving the exposed faces of the supported crystals over the abrasive surface in unison. Regardless of the particular manual method employed, each crystal is, during the finish grindgrinding, be they semi-mechanical or manual, result in excessive waste due to crystal breakage.

It is an object of the present invention,'therefore, to provide an improved method of producing finished piezoelectric crystals which substantially obviates al1 ofthe disadvantages of conventional grinding methods previously in use. 1

It is another object of the present invention to provide an improved method of finishing the face -surfaces of piezoelectric crystals to predetermined dimensional and contour standards in batches and with a minimum expenditure 'of manual labor.

According to another object of the invention, the face grinding actionis obtained by tumbling the crystals in the presence of a loose abrasive material on a variable time basis and in an improved manner such that the desired face contouring is realized.

In accordance with another object of the illvention, an improved method of abrading the face surfaces of the crystals is provided, such that the original face contour of the'crystals is retained as the abrading action-proceeds, or the abrading action is concentrated in the cen-' tral zones of .the face surfaces, thus tending to It is yet another object of the invention to provide an improved method of so agitating a mixture of loose abrasive material and crystals that the face surfaces of the crystals are only intermittently abraded as the agitation of the mixture proceeds.

According to a further object of the invention,

. a controlled method of finish grinding piezoelectric crystals is provided in which the finish ground crystals are produced with a minimum expenditure of manual labor and on a variable time basis to conform to a pro-established frequency standard.

It is also an object of the present invention to provide an improved method of finish grinding piezoelectric crystals in batches at high speeds and with a minimum expenditure of manual labor In accordance with still another obiect of the invention, the speed of agitation of any given mixture of crystals and abrasive material is so controlled that first the faces and then the edges of the crystals are abraded, or vice versa, all without removal of the crystals from the abrasive material.

It is a further object of the invention to utilize coarse cheap abrasive materials in finish grinding piezoelectric crystals, while obviating the dif-' ficulties accompanying the use of such materials in practicing conventional grinding methods, and yet producing ground surfaces which are more ticularly to Figs. 1 and 2 thereof, the crystal highly polished and contain fewer scratches than I the surfaces obtained by using optical flower abrasives in the practice of conventional high contact pressure manual grinding methods.

It is a still further'obiect of the invention to provide an improved method of finish grinding piezoelectric crystals wherein low contact pressures are at all times maintained between the crystal surfaces and the abrasive material.

According to still another important object of the invention, the finishing of the crystals to the desired resonant frequency is in part accomplished by etching the face surfaces thereof concurrently'with the finish grinding of the crystals.

In accordance with still another and more specific object of the invention the finishing of a batch of crystals is carried out in a plurality'of steps in each of which the crystals are agitated in the presence of a loose abrasive material, and in the last of which the crystals are concurrently abraded and etched by agitating the same in a mixture of abrading material and etching solution, whereby the surfaces of the finished crystals are substantially free from disoriented, deformed or cracked crystal particles and are also free from imbedded or attached loose foreign materials.

A further object of the invention is to obtain concurrent etching and abrading of the crystal faces without expending the etching solution on the abrasive material.

The invention, both as to its organization and grinding apparatus there illustrated comprises a horizontal base l0 having two upstanding tubular brackets ii and i2 rigidly supported thereon in spaced apart relationship. At the upper ends thereof, these two brackets pivotally support two bearing members I1 and II within which a drive shaft I8 is journalled. More specifically, the tubular bracket is split downwardly from its upper end along one side thereof, and is adapted to receive the shaft of a bearing yoke l3, which is provided with legs for supporting the bearing member ll therebetween. The shaft portion of the yoke I 3 is clamped within the split end of the bracket H by means of two lugs extending outwardly from this bracket upon opposite sides of the slot formed therein and a clamping screw llc which extends through one of the lugs and is threaded into the other lug. At their upper ends, the legs of the yoke H are threaded to receive pivot bolts Na and lib having inner opposed ends which enter deadended pivot holes provided at opposed points around the bearing member l'l. Lock nuts threaded onto the pivot bolts and adapted for clamping engagement with the adjacent leg ends of the yoke II are utilized to hold the bolts Ho and I3!) in set positions. With this supporting structure, the bearing member II is mounted for universal movement relative to the base ill. By means of an identical supporting structure comprising the bearing yoke supported within the upper end of the tubular bracket I2, the bearing member I! is likewise mounted for universal movement with respect to this bracket and the base M. This bearing mounting arrangement permits self alignment of the shaft l8 within the bearing surfaces of the two bearing members I! and I8 without exacting adjustment of the positions of these two members. The shaft 16 functions to support two tumbling jars 25a and 25b, each of which is .of

square cross sectional configuration along the body thereof and has a smaller neck portion suit ably threaded at the end to receive an internally threaded cover 24. To support these jars, face plate members Isa and lilb are fixedly mounted upon opposite ends of the shaft It for rotation therewith. These members, in the engagement of the hub portions thereof with the ends of the bearing members H and I8 also serve to prevent axial movement of the shaft IS. The face plate portion of the members Ho and I9!) respectively carries four outwardly projecting, angularly spaced apart, resilient arms 20 and 20b which in combination define nests for receiving tumbling jars 25a and 25b. The four arms 20a, for example, forming one cup receiving nest may be bent over from an angular base 2 la, so that a one-piece construction is provided, and the angular base Ma may be suitably secured to the outer face surface of the associated member 19a by spot welding the inner edge thereof to the adjacent plate surface. If the tumbling jar 25a, for exam pie, is within the resilient arms 20a, the bent over and portions 22a of the arms engage the base of the Jar neck to hold the jar in place. If desired, a rubber band 23a may be used to increase and 25b about the long axes thereof, a motor 26 is provided which is rigidly mounted upon the.

base L This motor is arranged to drive the shaft it through a speed reducing gear train which comprises a worm gear 29 carried by the shaft l6 and engaging a worm 28 mounted for rotation with the motor rotor shaft 21. Although the motor 28 may be of any desired adjustable speed type, it is preferably an adjustable speed direct current motor, and may be connected for energization from a direct current source indicated by the bracketed terminals 32. Anvadjustable rheostat comprising the resistor element 30 and an adjustable wiper arm II, i provided in one side of the circuit for energizing the motor 26 in order that the speed of operation of this motor may, within limits, be varied as desired.

In utilizing the above described apparatus to wherein the disposition of the mass within the tumbling Jar at successive angular positions of the Jar is shown. From a comparison of the mass distribution as shown by these figures, it will be observed that a mass breakdown occurs between th Fig. 5 and Fig. 6 positions of the jar, the moving part of the massbeing generally indicated as overlying the division line 8 shown in Fig. 5, and that after each breakdown of the mass the upper surface thereof is leveled and then tilted in the direction of rotation of the Jar.

During the sliding movement of only the upper surface part of the mass, those crystals which are included in this mass part are moved relative to the abrasive material and are subjected to a low contact pressure grinding operation at the face surfaces thereof. At this point it may be noted that after rotation of the tumbling jar at a definite speed is well established, the crystals tend so to position themselves within the mass that the face surfaces are parallel to the axis of I rotation of the tumbling Jar. During each practice the present improved grinding methods.

piezoelectric quartz crystals which have been cut from the crystal stock and then machine ground to rectangular dimensions slightly larger than those desired, are mixed with a charge of loose abrasive material and a liquid cleaning agent or an etching solution in one or both of the two jars 25a and 25b, following which the jars with the covers 24 screwed over the opened ends thereof are inserted between the arms a and 20b. Operation of the motor N to rotate the tumbling Jars 25a. and 25b at the speed established by the setting of the rheostat-wiper arm Il may now be initiated. At this point it may be noted that when a tumbling jar having given cross sectional dimensions is used, the desired face abrasion of the crystals is only obtained when the jar is rotated at a speed falling within a predetermined speed range. Thus when a square jar having side dimensions of approximately 7 inches is rotated at speeds of approximately 10 revolutions per minute and less. practically no relative movement occurs between the crystal blanks of the mass and the loose abrasive material with which these blanks are intermixed. As a result, the grinding action is excessively low. 'This lack of relative movement between the crystal blanks and the abrasive material and th resulting slowness of the abrasive action, may be attributed to the fact that as rotation of the Jar proceeds at such slow speeds, the entire mass of crystals, abrasive material and liquid cleaning agent within the Jar simply slides over the inner surface of the Jar with only a shifting of the cross sectional configuration of the mass. As the speed of rotation of the tumbling jar is increased, the mass of crystals, grinding material and liquid cleaning agent is frictionally carried up the leading side surface of the jar so that the upper surface of the mass becomes tilted with respect to the horizontal. When a predetermined tumbling jar speed of rotation is exceeded, the angle of inclination of the mass with respect to the horizontal exceeds the inherent angle of repose of the mass. When this critical angle of inclination is exceeded, the

breakdown of the mass, therefore, each crystal blank in the upper surface portion or moving part of the mass is disposed face downwardly against theabrasive material with the result that the abrasive contacted face surfaces thereof are abraded. It i also pointed out that during each breakdown of the mass in the manner described, the bottom part of the mass, 1. e. that below the line 9, remains substantially static in that very little relative movement occurs between the constituents thereof. Accordingly, those crystals which are disposed at the juncture 9 between the lower static part of the mass and the upper I moving part of the mass are moved relative to the abrasive material at both face surfaces thereof with the result that both said surfaces are ground. It has also been observed that those crystals which appear at the surface of the sliding portion of the mass move toward the bottom upper surface part of the mass breaks away from action which occurs when the jar is rotating at a speed above the described-critical value is well illustrated in Figs. 3, 4, 5 and 6 of the drawings,

of the mass with greater rapidity than the abrasive material included in the moving mass part. During such relative movement between the sliding abrasive and the contacting face surfaces of the crystals riding over this abrasive, abrasion of the indicated crystal faces obviously occurs.

With the tumbling jar rotating at a constant speed above the described critical value, the above described breakdown of the mass within the jar occurs intermittently at periodic intervals. Thus with a four sided jar operating at a speed of from twenty-five to forty revolutions per minute. it has been found that the mass within the Jar breaks down at a rate of approximately three times for each revolution of the jar. During the periods separating the breakdown intervals the entire mass remains substantially static in that relatively little'relative movement occurs between the abrasive material and the crystals making up the mass. During such periods, therefore, practically no abrasion of the crystal surfaces is produced. At the end of each, mass breakdown operation, a new portion of the mass obviously occupies the leading position insofar asmovement of the mass is concerned. It will be understood, thereforethat successively different parts of the mass are slipped to the bottom of the pile during-successive breakdown intervals. Due to this continuous shift cluded in the sliding mass parts during successive breakdown operations. It will thus be apparent a that as the tumbling of the mass continues, each crystal describes an orbit which is entirely repetitious insofar as the downhill sliding part of the orbit is concerned and is somewhat repetitious insofar as movement of the crystal within the mass is concerned. In other words, rotation of the tumbling Jar causes the mixture of crystals, abrasive material and liquid cleaning agent to slowly flow, such that the crystals are caused to migrate about in and slide upon the abrasive material with exceeding low contact pressures therebetween. It will also be apparent that each crystal is face abraded intermittently and at a. rate which is considerably slower than the periodic breakdown rate of the mass. As each mass part sliding action ends, those crystals involved therein are tumbled end over end so that different face surfaces of each crystal are abraded as the tumbling proceeds. The desired change in position of each crystal required to insure the same average grinding of both faces thereof during the succeeding breakdown operations in which the crystal is involved, is further obtained by the changing position of the crystal within the mass during those intervals when it is buried in the static part of the mass and hence is not involved in the breakdown operations. It has been found that for a given tumbling period,the faces of all crystals of a given mixture are substantially uniformly abraded, indicating that on an average basis the rate and paths of circulation of the crystals through the upper and lower parts of the mass are about the same for all crystals.

' More specifically considered, the finish grinding of a given batch of quartz crystals is carried out in two or more steps. During the first or primary step, the crystals are ground to approach the particular resonant frequency characteristic desired for each crystal. In this regard it will be understood that the crystal blanks as initially cut are lap ground to such dimensions that the resonant frequency of each crystal is well below the particular resonant frequency which is desired. During the primary grinding step, therefore, the extent of grinding is limited not to exceed an amount which will bring any of the crystals to a resonant frequency higher than that desired. It has been found that the primary grinding step may be satisfactorily carried out by mixing the crystal blanks with a charge of abrasive material having the following specifications. The tumbling jar 25a, for example, is filled full of body material granules consisting of 3--5 or 58 mesh garnet, fused aluminum oxide or Alundum, to which is added 50 to 60 cc. of silicon carbide of any mesh between 180 and 250. .To this mixture water and Aerosol are added until the jar is 'A; full of the abrasive material and a of 1 per cent Aerosol solution. After the charge of grinding material has been prepared in the manner just explained, the crystal blanks may be introduced into the mixture and tumbled for a predetermined time interval at a pro-established tumbling rate. As the tumbling operation proceeds,, the coarse body material enhances the contact pressure obtained between the crystal faces and the abrasive material, and hence acts to speed up the grinding operation. The oil wetting agent, i. e. the Aerosol solution, acts'to break down and remove any oil deposits or films which may be present upon the crystal faces as the tumbling action proceeds. This cleaning action is enhanced by the action of the body material and abrasive powder, particularly the former, in scrubbing the crystal surfaces. The end result is that the surfaces of the crystals are thoroughly cleaned as the face grinding proceeds.

After the primary grinding for the selected time intervalis'completed, the crystals are segregated from the-mixture, are rinsed or washed in water. dried, measured to determine the resonant frequencies thereof, and are grouped or classified according to their measured resonant frequencies. In the usual case, certain of the crystals in the batch will be found to have been ground to resonant frequencies which are so close to the desired predetermined frequency as to require no further grinding. Theremaining crystals, 1. e. those having resonant frequencies well belowthe desired value, are subjected to further grinding. To this end, the non-acceptable crystals are re-mixed with the original charge of body material to which a new charge of silicon carbide and Aerosol solution is added, the used charge of silicon carbide and Aerosol solution having been washed out of the body material at the end of the first grinding operation. The new mixture is now tumbled at the same speed for a second predetermined time interval following which the crystals are again separated from the grinding material, washed and re-classifled as to frequency. This step by step process of grinding and testing is repeated until the predominant portion of the crystals have been ground to the desired resonant frequency, it being understood that those crystals having acceptable resonant frequencies are removed from the'batch of crystals at the end of each grinding operation.

It has been found that the above described method of grinding produces an exact and predictable frequency increase for crystals of a given size on a time basis with a frequency variation of less than ten per cent. Thus, when using v the same body granules and operating at the same tumbling speed of 35 revolutions per minute, a frequency change of from 5.5 kilocycles per hour to 5.8 kilocycles per hour has been obtained forty times in succession in the grinding of 4.3 megacycle crystals. Using this same tumbling speed, other frequency changes which have been obtained with the same degree of reliability on crystals of the same and different sizes are outlined in the following table:

ody Frequen Crystal frequency granule Abrasive increase size per hour 4. 3 megacycles 4-6 220 silicon carbide. 5035 cycles 4.3 megacyclea- 3-5 220 silicon carbide. 5500 cycles. 4.3 megacycles. H None 650 cycles. 6.8 megacyclss" 5-8 None 725 cycles.

6.8 megacyclea. 5-8 220 silicon carbide 3800 cycles.

6.8 megacycles 4-6 220 silicon carbide.. 6600 cycles.

8.1 megacycles 5 -8 220 silicon carbido 8500 cycles.

With this data available as to the degree of frequency change per unit interval of grinding time, for crystals of a given size, an alternative method of grinding procedure may be employed. This alternative process is carried out by classifylng the crystal blanks according to their resonant frequencies before the grinding is started. More specifically, the crystal blanks are lap ground to have resonant frequencies ranging froml5 to 65 kilocycles less than the particular desired value, and are then classified in groups according to the resonant frequenciesthereof so that the resonant frequencies of the crystals in each group are not more than 10 kilocycles apart and the frequency bands of the different groups are non-overlapping. After the classification is completed, those crystals that need the most grinding to raise the resonant frequencies thereof to the desired value are first ground for a predetermined time interval. At the end of this interval, the crystals in the next adjacent 10 kilocycle band are added to the mix without removalof the crystals of the first group, and the resonant frequencies thereof to the. particular desired value. At this time the predominant portion of the crystals are from threeto tenvkilocycles lower than the desired. frequency. The

crystals of the batch are now reclassified according to the frequencies thereof, fOllOWing whicn the grinding procedure of successively adding the crystals of different groups to the grinding mix is repeated. At the end of the first grinding operation, approximately five per cent of the crystals will be found to have the desired resonant frequencies and may be removed from the batch. At the end of the second grinding operation approximately eighty per cent of the crystals will be found to have the desired resonant frequency within permissible tolerances. repeated a third time on a controlled time basis, another ten per cent of the crystals will be found to have the desired resonant frequencies.

An alternative procedure which may be followed in finish grinding abatch of unfinished crystals is that of first preliminarily grinding all it is removed from the batch, and that on an average basis the grinding increments are shortened as the crystals are ground closer and closer to the desired resonant frequency.

From the above explanation it will be understood that as the successive grinding operations are performed, the crystals involved therein approach with increasing nearness the particular desired frequency. In order to prevent over grinding, it may be desirable to reduce the grinding rate during the final grinding operations.

avoided.

If the process ,is

of the unground crystals in the above described manner for a given time interval calculated not to raise the resonant frequencies of any of the crystals above the desired frequency, and then classifyiiig the crystals in groups according to their measured resonant frequencies. The crystals of the respective groups are then separately mixed with different charges of abrasive material and liquid cleaning agent'indifferent tumbling jars. Each mix is next separately tumbled or agitated at a predetermined tumbling rate for a particular time interval which is dependent upon the difference between the average resonant frequency of the class or group of crystals and the desired resonant frequency.-'Assuming,v for eX-- ample, that there are threedifferent groups of crystals which respectively contain crystals of fied with a'lesser frequency difference between the average resonant frequencies otthe different grouns. and a ain separatelv ground in groups for different increments of time. This step-bystep process may be'repeated as many times as may be necessary to bring .the resonant frequencies of the crystals within the desired frequency tolerance range. It will be understood. however, that as each crystal is brought within this range This may conveniently be done by progressively decreasing the size. of the body material used in the grinding mix. As will be evident from the above table, by, decreasing the size of the body material granules, the frequency change produced per unit of grinding time is correspondingly reduced. Accordingly by decreasing the body material granules used in the successive grinding operations, the final grinding operations may be much more accurately controlled on a time basis, and hence over grinding may be If exceedingly slow grinding is desired, the abrasive powder or cutting material may be omitted entirely from the mixture.

The size of the body material granules emf with increasing crystal frequency. Hence, high frequency crystals are much more fragile and =likely to break during the grinding operations than are low frequency crystals. This tendency toward breakage 'of the crystals is, moreover, in-' creased with an increase inthe size of the body granules employed. Also, and as will be evident from the above table, for a given'grinding mix the frequency change: produced per unit of, grinding time increases rapidly with increasing resonant frequencies of the crystals. It is preferable, therefore, to usesmaller' granules of body material in the grinding of ultra high frequency crystals than are employed in the grinding of the lower frequency crystals. On an average .basis, from three to five mesh granules of body material have been found to be entirely satisfactory in the grinding of crystals having desired resonant frequencies lower than 5 megacycles. It has also been found that-body material consisting of 4-8 mesh granules is entirely satisfactory for use in the grinding of crystals having desired resonant frequencies above 5 megacycles. More generally stated, in the grinding of crystals to desired frequencies of from 4 to 8.5 meg- :tially affecting therate of cutting or the character of the ground surface produced. In a proper mix, the cutting material may range from 3 to 15 per cent, by weight, of.the body material.

One important aspect of the present invention relates to thestep of concurrently abrading and etching the crystal faces in order to accelerate somewhat the rate of frequency change and to I insure removal of all loose particles from the surfaces of the crystals. This step has been found to be particularly advantageous in moving the crystals the last five kilocycles toward the desired resonant frequency, or in other words when used as the final finishing step or steps in practicing any one of the above-described methods. It may conveniently be practicedby adding one suitable etching salt to one quart of abrading material consisting of well worn garnet or aluminum oxide which may be of a size ranging from 6 to 30 mesh. These ingredients may be mixed with one quart of water in the tumbling jar, following which the crystals may be added to the mixture and the tumbling action initiated. The etching solution formed by the mixture of etching salt and water has the effect of eating away the surfaces of the crystals to decrease the thick-' ness of each crystal and thus increase its frequency. Also, the solution is highly effective in removing foreign matter and particles from the crystal surfaces. The gentle abrading action of.

, actor of the grinding aggregate or body material, 1. e. the mass of each granule as determined by upon to produce the desired frequency change.

Further, the etching solution rapidly eats away loose crystal particles or other particles of foreign material which might otherwise become imbedded in the crystal pores. As a result, the finished crystals, after having been removed from the mixture, rinsed and dried are absolutely clean and the surfaces thereof are entirely free from disoriented, deformed or cracked segments. Accordingly they have ;a high degree of stability under extreme temperature and humidity conditions and over an extended aging period. Although the etching salt may be of any desired type, it is preferably an ammonium bi-fluoride salt, commercially sold under the trade names of Safety Etch," Quartz Etch and Frequency Etch. Alternatively, a strong caustic solution or a sulphuric acid, potassium dichromate-mixture may be used in' making up the etching solution.

Any'of the etching fluorides, such, for example, as hydrofluoric acid, may likewise be used as the etching media. In this connection it is noted that the body and abrasive material granules may be so chosen that the etching chemicals will not act upon them, or will do so very slowly, thus making the etching action on the crystal surfaces much more effective. Thus, both the body and cutting material granules may be formed of stainless steel. or tungsten carbide, which are only acted upon very slowly by the usual etching chemicals. Preferably, however,

this is accomplished by using body material con-.

sisting of bronze, copper or lead granules, which are substantially non-reactive with the etching solution, and by using a cutting material, when required, consisting of tungsten carbide or boron carbide. In this regard, it will be understood the size and density of the granule, is a relatively small factor insofar as the production of a desired crystal face configuration is concerned. Primarily, this configuration is determined by the tumbling speed employed in effecting the abrading action. Thus at verylow tumbling speeds below a value of approximately 10 revolutions per minute, practically no'relative movement occurs between the blanks and the abrasive material and hence an almost negligible grinding action is obtained. At increased tumbling speeds, i. e. those in excess of the critical speed at which periodic mass breakdown occurs, the amount of face grinding per unit of grinding time becomes appreciableand the grinding occurs substantially uniformly over the crystal faces, with the result that'the face surfaces tend to retain their original .contours as the grinding proceeds. At still higher tumbling speeds, the abrading action is increasingly effective toward the center of each crystal face. Accordingly a concave configuration is im-v the presence of corresponding charges of abradthat in certain cases it is desirable to use in conjunction with the etching solution an abrasive material which consists of both the relatively large body granules and the relatively fine cut ting material. By using abrasive materials of the specific. character referred to, the etching solution does not expend itself upon the abrasive materials and hence maintains its effectiveness in acting upon the crystal surfaces. Moreover, the effectiveness of the abrasive materials is not impaired by the action of the etching solution thereon.

As previously indicated, the change in contour of the crystal faces occurring during the finishing operations is definitely a function of the ing material and in tumbling Jars of the same cross sectional dimensions. The curves A-l, A--2 and A3 illustrate the change in crystal face contour which occurs incident to the tumbling of one of the typical crystals at a tumbling speed of 35 revolutions per minute for intervals of 12, 24 and 36 hours. From a comparison of these three contour curves it will be observed that at the identified tumbling speed of 35 revolutions per minute, the predominant portion of the grinding occurs at the centers of the crystal faces such that the initially convex face of the particular crystal in question is gradually flattened, and as the grinding operation continues a. slightly concave contour is imparted thereto. The three contour curves C-l, C2 and C--3 illustrate the face configurations which are obtained by subjecting a typical crystal to the grinding operation for intervals of 12, 24 and 36 hours at a tumbling speed of 40 revolutions per minute. These three curves clearly indicate, when compared with the corresponding curves A-l, A-2 and A3, that the increase in tumbling speed results in an increased concentration of the grinding action at the centers of the crystal faces such that these faces more rapidly tend to assume concave 0on figurations as the grinding action proceeds. From a study of the illustrated curves, it will be clearly apparent that at some tumbling speed below the tumbling rate 35 revolutions per minute a substantially uniform face abrading action will be obtained, such that the original face contours of the crystals will be retained as the grinding operation proceeds. When a four sided tumbiing jar having side dimensions of 7 /2 inches is used, the tumbling speed at which uniform face grinding occurs has been determined to be approximately 30 revolutions per minute. As the ,387,142 r defined, the change in face contour is a function of the number of mass breakdowns which are produced in a given time interval. Thus it has been found that over the entire tumbling speed range of from to 40 revolutions per minute for a square tumbling jar having 7 inch side dimensions, approximately three mass breakdowns occur during each revolution of the jar. From this it may be readily determined that at a mass breakdown rate of approximately 90 breakdowns per minute substantially uniform grinding is obtained. On ,the other hand, at breakdown rates ranging from 90 to 120 breakdowns per minute uniform to concave face grinding is obtained. From this .data'the Jar configuration and tumbling speed reqiflred to produce a desired face contour change may readily be determined with only asmall amount of 'experimentation.

From the above explanation it will be apparent that by selecting a group of crystals havingsubstantially matching face contours, mixing the crystals with a predetermined abrasive material of the character described, and tumbling the overall mixture for a predetermined time interval, a uniform and predictable change in the face contours of the crystals will be obtained.

On an experimental basis, it has been found that in the grinding of piezoelectric crystals to specific desired resonant frequencies in the manner explained above, tumbling speeds of from 25 to 40 revolutions per minute may be satisfactorily employed when a square tumbling jar is used having 7 inch side dimensions. This tumbling speed range is permissible because of the fact that, within the period normally required to bring the crystals up to the desired frequencies, the face contour change produced at any tumbling speed within this range is not intolerable. Normally, the crystal blanks have somewhat convex face contours after they are lapped to the approximate desired thicknesses. Since substantially fiat face contours are preferred in the finished crystals, it is desirable to select a tumbling speed at which a tendency to produce concave face contours is provided. With a jar of the size indicated, it has been found that a tumbling spe'ed'of approximately revolutions per minute will provide the required concaving tendency necessary to flatten the usual crystal blank during the overall grinding interval required to raise the resonant frequency of the crystal to the desired value.

At tumbling speeds in excess of 40 to revolutions per minute the above described mass break- 7 down action is obliterated with the result that the crystal blanks are knocked about within the tumbling jar to bring"thee"dges thereof into contact with the body granules and the abrasive material. As a result, thgmajgrlportion of the grinding action occurs atthe crystal edges and very little face abrasion of the crystals is-produced. The end result is that at tumbling speeds in excess of approximately 45 revolutions per minute, the edges of the crystal blanks are beveled or chamfered without appreciable face grinding. This beveling action is extremely desirable since it serves to remove edge irregularities from the crystal blanks, with the result that the activity of each crystal is increased on a time controlled basis and the further result that dips or valleys are removed from the temperature-activity characteristic curve of each crystal. This observed edge grinding at high. tumbling speeds may conveniently be utilized to impart the desired activity characteristics to a batch of crystals before the crystals have been face ground to the desired resonant frequency. Thus thecrystals-of a batch may be mixed with an abrasive charge in the manner described above and tumbled at a speed in excess of revolutions per minute for the purpose of grinding the edges of the crystals. The high speed tumbling may now be continued for a selected predetermined time interval calculated to impart the desired activity characteristics to a certain portion of the crystals being ground. Thereafter, the crystals may be subjected to low speed tumbling in the manner described above. for the purpose of face grinding the crystals to the desired resonant frequency. The described speed tumbling, if desired.

Although the methods have been described as utilizing silicon carbide as the grinding media, it will be understood that other abrasive materials, such, for example, as boron carbide.

Allmdum, ngsten carbide, molybdenum carbide and tantalum carbide may be used. The only apparent limitation upon the type of abrasive material which may be used, is that the material from which the piezoelectric crystals are formed be less hard than the abrasive material. It will also be understood that by utilizing the present improved methods of finish grinding piezoelectric crystals, the cost of the finish grinding operation is reduced to an exceedingly low figure. On a relative basis, it may be noted that the cost of hand grinding a crystal ranges from seventeen totwenty-five times the cost of finish grinding a crystal when theabovedescribed improved methods are employed. The methods disclosed herein are also characterized by the additional important feature thatthe finish grinding operation may be carried out with very little waste, in that only a small percentage of the finished crystals are rejected because of being broken or overground. When hand grinding methods are used, on the other hand, great skill is required in order to prevent crystal breakage and to 'prevent the crystals from being overground or, in other words, ground to a resonant frequency higher than the desired resonant freouency. Another advantage of the present improved methods relates to the fact that in practicing these methods it is practically impossible to produce scratches or other blemishes in the crystal faces even though much coarser grinding abrasives are used in practicing the methods than are employed in practicing conventional manual methods; This is due to the, fact that the contact pressures between the abrasive material and the crystal surfaces are relatively every low dring abrasion of these surfaces. Thus the total weight acting on the entire crystal area of each crystal to produce contact pressure 'between the abrasive material and the crystal face will not exceed '7 or 8 grams. In manual grinding. on the other hand, the total weight actin on each crystal face area is usually in excess of two pounds during each excursion of the crystal across the abrasive surface. As a result. if a frequency. 1

ual sweep of the crystal across an abraslve finish- I in plate by a finisher is about equal to-that produced by utilizing the present improved method with a slow frequency moving abrasive to grind the crystal for one hour. This indicates the difference between the contact pressure employed modifications may be made therein, which are within the true spirit and scope of the invention as defined in the appended claims.

Iclaim: l 1. The method of finishing piezoelectric crystals to establish a predetermined resonant frequency characteristic for each crystal, which comprises mixing the crystals with a charge of loose abrasive material and an etching solution, and agitating the mixture, thereby concurrently to abrade and etch the faces of the crystals and thus increase the resonant frequencies thereof at substantially the same predetermined rate.

2. The process of producing piezoelectric crystals having a desired resonant frequency from unfinished crystals of lower frequency, which comprises the steps of mixing the unfinished crystals with a mass of loose abrasive material and an etching solution which eats away the crystal surfaces, and agitating said mixture to produce flowing of the mixture such that the crystals migrate about in and slide upon the abrasive material with low pressure contact therebetween, whereby the faces of the crystals'are concurrently abraded and etched to increase the resonant frequencies thereof-at substantially the .same predetermined rate.

3. The process of producing piezoelectric crys- I tals having a desired resonant frequency from crystals oflower frequency, which comprises the steps of mixing the unfinished crystals'with a mass'of loose abrasive material, agitating the mixture to produce flowing of the mixture such that the crystals migrate about in and slide upon the abrasive material with low contact pressure therebetween, thereby to raise the resonant frequencies of the crystals toward said'desired resonant frequency, separating the crystals from the mixture, remixing the crystals with a mass of loose abrasive material and an etching solution which eats away, the crystal surfaces, and

' agitating said last-named mixture, whereby the slide upon theabrasive material with low contact pressure therebetween, thereby to raise the resonant frequencies of the crystals toward said desired resonant frequency, separating the crys tals from the mixture, remixing the crystals with a mass of relatively large abrasive'bodies-and an etching solution, and agitating said lastnamedrnixture, whereby the faces of the crystals are concurrentlyabraded and etched to further increase the resonant frequencies of the crystals toward said desired resonant frequency- 5. The process of producingpiezoelectric crystalshaving a desired resonant frequency from unfinished crystals of lower frequency, which comprises the steps of mixing the unfinished crystals with a mass of abrading material comprising relatively large carrier bodies which are of a size between 3 and 10 mesh and a relatively fineigran'ular cutting material, agitating the mixture to slowly fiow the mixture so that the C ystals are caused to migrate about in and slide upon the abrasive material with-low contact pressure therebetween, thereby to raise the resonant frequencies of the crystals toward said desired resonant frequency, separating the crystals from the mixture, remixing the crystals with a mass of relatively large abrasive bodies and an etching solution, and agitating said lastnamed mixture, whereby the faces of the crystals are concurrently abraded and .etchedto further increase the resonant frequencies of the crystals toward said desired resonant frequency.

6. The process of producing piezoelectric crystals having a desired resonant frequency-from unfinished crystals of lower frequency, which comprises the steps of mixing the unfinished crystals with a mass of abrading material comprising relatively large carrier bodies which are of a size between 3 and 10 mesh and" a relatively 40 1:16 granular cutting material, agitating the mixture to slowly fiow the mixture so that the crystals are caused to migrate about in and slide .upon the abrasive material with low contact pressure therebetween, thereby to raise the resonant frequencies of the crystals toward said desired resonant frequency, separating the crystals from the mixture,.remixing the crystals with a mass of relatively large abrasive bodies which are of a size between 6 and 30 mesh and an etching solution, and agitating said last named mixture to slowly fiow the same so that the crystals are caused to migrate about in and slide upon the abrasive material with low contact pressure therebetween, whereby the faces of the crystals are concurrentlyabradedand etched to further increase the resonant frequencies thereof toward said desired resonant frequencyand are at the comprises the steps of mixing the unfinished.

faces 'of the'crystals are concurrentlyabradedv and etched to further increase the resonant frequencies thereof toward said desired resonant 4. The process of producing piezoelectric crystals having a desired resonant frequency from unfinished crystals of lower frequency, which comprises the steps of mixing the unfinished crystals with a mass of abrading material comprising relatively large carrier bodies and a relatively fine granular abrasive material, agitating the mixture to slowly flow the mixture so that the crystals are caused to migrate about in and same time washed and cleaned. v r 7. The process of producingpiezoelectric crystals having ajdesired resonant frequency from.

unfinished crystals of lower frequency, which crystalswith a mass of abrasive bodies which are of a size between}; and 30 mesh and an etching solution, and agitating the mixture to slowly fiow the same so that the crystals are caused to migrate about in and slide upon the abrasive bodies with low contact pressures therebetween,

whereby the faces of the crystals-are concurrent-l ly abraded and etched to increase the resonant frequencies thereof towardthe desired resonantfrequency and are at the same time washed and cleaned.

8. Th method of finishing piezoelectric crystals to establish a predetermined resonant frequency characteristic for" each crystal, which comprises the step ofconcurrently and substantially uniformly abrading and etching the faces of a batch of said crystals at a controlled predetermined rate to produce a change in the resonant frequency of each crystal which is pre-' dictable as a function of the abrading and etching time. v

9. The method of finishing piezoelectric crystals to establish a predetermined resonant frequency for each crystal, which comprises mixing the crystals with an etching solution and loose I abrasive material which at least in part is not acted upon by theetching solution, and agitating the mixture, thereby concurrently to abrade and etch the faces of the crystals and thus increase the resonant frequencies thereof toward said predetermined resonant frequency.

11. The 'method of producing piezoelectric crystals having a desired frequency from unfinished crystals of lower frequency, which comprises mixing the unfinished crystals with an etching solution and with loose abrasive material comprising a relatively fine granular cutting material and relatively large metal bodies which are substantially non-reactive with the etching solution, and agitating the mixture, thereby concurrently to abrade and etch the faces of the crystals and thus increase the resonant frequencies thereof toward said desired value.

12. The process of producing piezoelectric crystals having a desired resonant frequency from unfinished crystals of lower frequency, which comprises the steps of mixing the unfinished crystals with a mass of loose abrasive material,

" agitating the mixture to produce flowing of the 10. The method of producing piezoel ctric crystals having a desired frequency from unfinished crystals of lower frequency, which comprises mixing the unfinished crystals with an etching solution which eats away the crystal surfaces and with loose abrasive'material which at least in part is not substantially acted upon by the etching solution, and agitating the mixture, thereby concurrently to abrarie and etch the faces of the crystals and thus increase the resonant frequencies thereof toward said desired value.

mixture such that the crystals migrate about in and slide upon the abrasive material with low contact pressure therebetween, thereby to abrade the crystal faces and thus increase the resonant frequencies thereof toward said desired resonant frequency; removing the crystals from the abrasive material, and etching the crystal faces, thereby further to increase the resonant frequencies of said crystals toward said desired resonant frequency.

HAL F. FRUTH. 

